It is a well known problem that a hearing aid can become unstable and howl when loop gain exceeds 1. The (open) loop gain is a product of the gain in the hearing aid and the coupling between the receiver (speaker) and microphone, primarily, but not exclusively, through a vent in the earpiece. The vent is generally inserted in the earpiece of hearing aids so as to avoid occlusion. The coupling between the receiver and microphone is called the external or physical or acoustical feedback path and may have other origins than a deliberately arranged vent, e.g. mechanical coupling between various parts of the earpiece, etc.
Two methods can be used to help in situations where loop gain gets too high: Either by subtracting an estimate of the external feedback from the input signal (the microphone signal) or by reducing the gain in the hearing aid. The first method is used in so-called Dynamic Feedback Cancellation (DFC) or Anti-Feedback (AFB) systems, these terms are used interchangeably in the present application. This method has the advantage that the loop gain can exceed 1 without howls, meaning that the hearing impaired can get more gain or a larger vent compared to a situation without a DFC/AFB-system. A schematic illustration of a hearing aid system comprising a forward path, an acoustic feedback path and an electric feedback cancellation path is shown in FIG. 1b. The second method is sometimes used in the fitting situation, where the external feedback is measured and the maximum allowable gain is adjusted (‘the feedback manager’, FBM). But this is typically a one-time (offline) measurement, possibly performed by a technician, such as an audiologist, typically using specially adapted equipment.
U.S. Pat. No. 5,619,580 describes a hearing aid with digital, electronic compensation for acoustic feedback comprising a digital compensation circuit, including an adjustable digital filter and a first part, which monitors the loop gain and regulates the hearing aid amplification, so that the loop gain is less than a constant K, and a second part, which carries out a statistical evaluation of the filter coefficients, and changes the feedback function in accordance with this evaluation.
U.S. Pat. No. 6,219,427 deals with a digital hearing aid comprising a feedback cancellation system in the form of a cascade of two adaptive filters, a first filter for modelling near constant factors in the physical feedback path, and a second, quickly varying, filter for modelling variable factors in the feedback path, the first filter varying substantially slower than the second filter.
Published PCT-application WO 2006/063624 describes a hearing aid comprising a processor for amplifying an electrical input signal, an adaptive feedback suppression filter and a feedback model gain estimator that determines an upper processor gain limit based on inputs from the microphone, the adaptive filter, and the output form the processor.
EP 1 191 814 A1 deals with a hearing aid with an adaptive filter for suppression of acoustic feedback and a controller that is adapted to compensate for acoustic feedback by determination of a first parameter of an acoustic feedback loop of the hearing aid and adjustment of a second parameter of the hearing aid in response to the first parameter whereby generation of undesired sounds is substantially avoided.